What is a Compensating Brass Instrument?

The term compensating is one that is often referred to but not always fully understood. It is normally used with reference to lower brass instruments (eg. Euphoniums & Tubas) which are either 'compensating' or 'non-compensating' models. There is a common misconception that if an instrument has a 4th valve* it is compensating, but that is not necessarily the case. I'll try to explain exactly what it means as clearly and concisely as possible, although that's easier said than done!
The term compensating is one that is often referred to but not always fully understood. It is normally used with reference to lower brass instruments

Brass instrument - Wikipedia

Buckeye Brass and Winds - Cerveny Professional Compensating Euphonium

Available in gold lacquer or silver plate with gold trim., The Cambridge Classic non-compensating 4 valve Baritone Horn is the perfect step-up for 3

Cambridge Classic 4 Valve Non-Compensating Baritone Horn

Cambridge Classic 4/4 BBb Compensating Tuba - Euphoniums, baritone

1270 XO Euphonium

The Attitude, the Authority, the American Tuba Sound.Authoritative projections with vociferous sub tones - the Schiller American Heritage CC 5

American Heritage CC 5 Valve Piston Compensating Tuba - Brass

Jupiter 3+1 Valve Professional Compensating Euphonium - Silver-Plated

Paxman - Professional Model 33 Bb/F (A-+) Compensating Double

Euphonium Landmark Academy

Besson Compensating Euphonium

Trivia:The Compensating System for the Euphonium - Musical Instrument Guide - Yamaha Corporation